The Effect of Stormwater Discharge on the Nearshore Benthic Environment of Inner Wellington Harbour

Abstract

To investigate the spatial and temporal effects of stormwater discharge on the nearshore benthic environment of inner Wellington Harbour, sediment grain size, organic matter content, concentrations of As, Cr, Cu, Ni, Pb and Zn and the presence and abundance of the benthic macrobiota were measured at a number of stations at both stormwater outlets and at control sites. Correlations between the biological and the sedimentological and chemical variables were also investigated. In the vicinity of some outlets the sediment grain size distribution was altered and the sediment organic matter content was up to 5.5 times higher and metal concentrations were up to 11 times higher, than that at control sites. In general organic matter content and metal concentrations were highest close to an outlet and decreased with increasing distance seaward. With increasing distance from outlets the biological communities changed and there were significant differences in one or more of total, polychaete, arthropod and mollusc taxa, polychaete and mollusc individuals, diversity and the abundance of 12 taxa. The spatial extent of the impact of stormwater discharge on the benthic environment ranged from l0 to more than 45 metres from an outlet. Over time the largest range in organic matter content at an outlet was 6.1-42.8% while at a control it was 4-9%. For As, Cu, Pb and Zn the range in concentration over time was larger at stations closer to than those further from the outlet and larger at outlet than control site stations e.g. at 2 m in front of one outlet the Pb concentration range was 25l-1367 ppm, at 20 m from the outlet it was 15-22 ppm and at the associated control site 2 m station it was 15-20 ppm. Over time the direction of change in the biological community was different, and there were significant differences in the number of total, mollusc, polychaete and arthropod taxa and individuals and diversity, between that at the outlet and the associated control site. 27 taxa were more abundant at outlet than control sites and 45 taxa were more abundant at control than outlet sites over time. The biological community at outlet and control sites was best correlated to combinations of 2-5 of the sedimentological/chemical variables. Of these variables, organic matter content was the only commonly occurring variable at outlet sites and did not occur as one of the variables at the control sites. Where the organic matter content was >22% there were a low number of taxa and individuals, and with an increase in organic matter content there was a significant decrease in the number of total and arthropod taxa. The abundance of two taxa was significantly correlated to organic matter content and the abundance of six taxa was significantly correlated to metal concentrations.